Chloride the chloride ions remain presents in the water in the form of one or more of the compounds like CaCl2, MgCl2,NaCl and etc.
The estimation of chloride ions is generally, made by titrating the water sample against a standard solution of silver nitrate using potassium chromate as indicator. The added AgN03 precipitates chloride ions as insoluble white precipitate of silver chloride,
NaCl + AgNO3 —–> AgCl + NaNO2
Cl ˉ + Ag+ ——-> AgCl
and when all the chloride ions arc removed as silver chloride precipitate. the excess drop of silver nitrate reacts with potassium chromate forming silver chromate. which is red in color.
K2Cr04 + 2AgN03 —–> Ag2Cr04 + 2KCl
CrO4²ˉ + 2Ag+ —–> Ag2CrO4
The endpoint is,’tl1crefore, the change in color from bright yellow (due to K2CrO4,} to brick-red color (due to formation of Ag2CrCo4 )
Procedure: Take 100 ml of water sample. Add to it 2 to 3 drops of K2CrO4 indicator. Titrate against N/50 AgNO3 solution till the color changes from yellow to permanent brick-red.
Calculations: Let volume of N/50 AgNO3 used =V ml. Then chloride ions in water sample,
V X 35.5 X l,000,000/ 50 X 100 X 1,000 = 7.1 V
and the amount of chloride ions in water sample as CaCO3 equivalent,
= V X 50 X1,000,000 / 50 X 100 X 1,000 = 10V